1. Field
The present disclosure relates generally to processing data and in particular to processing data from responses of a structure to an input wave form. Still more particularly, the present disclosure relates to a method, apparatus, and computer usable program code for identifying anomalies in a structure.
2. Background
Composite and metallic aircraft structures may be susceptible to internal changes that may occur from fatigue, impacts, and other events or conditions. Composite materials typically have a minimal visual indication of these types of changes. As a result, an aircraft may be inspected to assess the integrity of the structure on a periodic basis or after visual indications of surface anomalies, such as, for example, dents and scratches.
For example, impacts to a structure, such as an aircraft, may occur during cargo loading and unloading. Inspections of the structure of an aircraft may be time-consuming and costly in terms of the time and skill needed to perform the inspection. Further, an airline may incur lost revenues from the aircraft being out of service.
Health monitoring techniques have been developed and used to monitor structures. These techniques often build the health monitoring systems into the structures. These health monitoring systems may be used to determine whether changes have occurred to these materials and structures over time.
Sudden changes in environment, such as, for example, electromagnetic effects, mechanical stresses, and other environmental effects, may affect the integrity of various materials and structures over time. By having health monitoring systems built into or associated with the structures to monitor the structures during use, appropriate measures and responses may be taken to prevent catastrophic failures and prolong the life span of these structures.
The monitoring of structures may include various non-destructive evaluation methods, such as ultrasonic testing or x-ray testing. Ultrasonic testing uses contact-based transducers to mechanically scan a structure. These distributed sensors and transmitters may be surface mounted on the structure or may be embedded in the structure to generate and propagate control of diagnostic signals into the structure being monitored.
A structural health monitoring system is based on using a transmitter and a sensor configuration to transmit waveforms at various frequency ranges and acquire data from the responses. Oftentimes, transducers may function both as a transmitter and a sensor. In some instances, a health monitoring system may only listen for signals.
The responses acquired from a structure may be compared to a baseline signal acquired at a prior time. The differences between a baseline signal and the response from a test signal may be characterized using dissimilarity indices. Commonly employed dissimilarity indices may have a number of drawbacks. These types of dissimilarity indices typically require that the excitation or test signals sent into the structure are identical for both the baseline and test measurements. Additionally, the range of values for dissimilarity indices is not easily bounded. Further, these types of indices also may have a path length dependent behavior.
Therefore, it would be advantageous to have a method and apparatus that overcomes one or more of the problems described above.